1. Field of the Invention
The present invention relates to a solid state relay in which the on/off switching operation of a power source is executed by a semiconductor power element (solid state semiconductor switching element) such as thyristor or triac and a load is turned on or off.
2. Prior Art Statement
Until now, a solid state relay as shown in FIG. 53 has been known as such a kind of solid state relay. In the diagram, reference numeral 511 denotes a control circuit and 512 indicates a power circuit. A series circuit of a light emitting element L such as a light emitting diode and a resistor R.sub.41 is connected between a pair of input terminals I.sub.1 and I.sub.2 in the control circuit 511.
Further first and second electrodes T.sub.1 and T.sub.2 of a semiconductor power element Q, e.g., triac are connected between power source terminals P.sub.1 and P.sub.2 in the power circuit 512. An anode and a cathode of a photo sensing element J, e.g., a photo thyristor are connected between a trigger terminal G of the triac Q and the first electrode T.sub.1 through a resistor R.sub.42. Further, a resistor R.sub.43 is connected between the second electrode T.sub.2 of the triac and the trigger terminal G.
In addition, a series circuit consisting of a resistor R.sub.44 for surge prevention and a capacitor C and an absorber consisting of a varistor Ba are connected in parallel between the first and second electrodes T.sub.1 and T.sub.2 of the triac Q. A series circuit consisting of an AC power source E and a load Z is connected between the power source terminals P.sub.1 and P.sub.2.
In the above construction, an input signal is applied between the input terminals I.sub.1 and I.sub.2. The photo sensing element J receives the light from the light emitting element L and is made operative. The photo sensing element J applies a trigger signal to the triac Q. Due to the activation of the triac Q, a current is supplied to the load Z through the triac Q.
Next, when the supply of the input signal is stopped and the light emitting element L is lit off, the photo sensing element J is made inoperative. No trigger signal is applied to the triac Q. The triac Q is turned off at the zero-cross point of a power source voltage, thereby stopping the current supply to the load Z.
In this manner, the on/off switching operation of the load current is performed by the triac Q in a contactless manner.
However, during the current supply to the load Z, if the portion between the power source terminals P.sub.1 and P.sub.2 is short-circuited due to an unexpected accident, an abnormal current flows through the triac Q, so that the triac Q is broken. In addition, even when a rated current is supplied, there is a case where the triac Q deteriorates over a period of time and is broken due to heat generation caused by a large current flowing through the triac Q.
Furthermore, in the conventional solid state relay, as shown in FIG. 54, the triac Q is fixed through an electric insulative plate 503 onto a heat radiating plate 502 fixed to a terminal base 501. Each terminal of the triac Q is electrically connected and fixed to a printed board 506 on which the control circuit 511 and power circuit 512 are assembled and to external terminals 507 and 508. Therefore, it is extremely difficult to replace only the triac Q and as a result, the whole assembly must be replaced which is uneconomical.
The power semiconductor element can be easily broken by the short-circuit accident or abnormal increase in temperature. The absorber element also deteriorates over a period of time. Therefore, when the power semiconductor element fails, it is important to exchange both the semiconductor element and the absorber for the purpose of improving reliability.
However, in the conventional relay, not only the power semiconductor element but also the absorber are assembled together with the other circuits on, for instance, the same circuit board. Therefore, when either one of them needs replacing, the whole assembly including the input circuit and the like must be replaced which is uneconomical. Also, even during replacement, detaching all of the wires can be complicated. Thus, there is a fear that erroneous wiring may be performed unless careful attention is paid to the rewiring work upon repair.
On the other hand, as shown in FIG. 55, there has been known a power source apparatus in which an AC input applied between a pair of input terminals is rectified by a rectifier and its output is smoothed by a smoothing circuit and is output to an output terminal. In the diagram, a rectifier 521 has a diode bridge circuit 522. One of AC terminals of the bridge circuit 522 is connected to an input terminal A.sub.1 through a parallel circuit of capacitors C.sub.51 and C.sub.52. The other AC terminal of the bridge circuit 522 is connected to the other input terminal A.sub.2 through a parallel circuit of resistors R.sub.51 and R.sub.52.
The varistor Ba is connected in parallel between the pair of input terminals A.sub.1 and A.sub.2. The varistor Ba constructs the absorber to absorb the surge. Zener diodes ZD.sub.21 and ZD.sub.22 are connected in parallel between DC output terminals of the rectifier 522. The Zener diodes ZD.sub.21 and ZD.sub.22 construct a clipper circuit to prevent the voltage which is full wave rectified and exceeds a specified value. Resistors R.sub.53 and R.sub.54 are respectively serially connected to these Zener diodes. Currents flowing through these Zener diodes are distributed with a good balance. The rectifier 521 is constructed in this manner.
A smoothing capacitor C.sub.53 is connected in parallel between the output terminals of the rectifier 521 and constructs a smoothing circuit 523 to smooth the output. The DC output smoothed by the smoothing circuit 523 is output to a pair of output terminals B.sub.1 and B.sub.2 to which the load Z is connected.
In the above construction, when an AC input is applied between the input terminals A.sub.1 and A.sub.2, the AC input is full wave rectified by the rectifier 521. The DC output of the rectifier 521 is smoothed by the smoothing circuit 523 and a current is supplied to the load Z through the output terminals B.sub.1 and B.sub.2.
However, according to this construction, if a smoothing efficiency of the output voltage of the smoothing circuit 523 is intended to be improved, the capacity of the capacitor C.sub.53 needs to be enlarged in association with the improvement of the smoothing efficiency. Therefore, the response speed of the output to the input in this kind of power source apparatus becomes slow. For instance, in the case of driving the load Z such as a solid state relay or the like in which a triac is assembled, there is a drawback such that the load Z cannot be switched on or off at a high speed.
On the other hand, since the response speed of the power source apparatus depends on an impedance of the load Z, there is a drawback such that the response speed of the power source apparatus must be individually measured for every load Z that is connected.
Further, since the smoothed output voltage includes a ripple component and changes in an analog-wise manner when the AC input is turned on or off, there is a drawback such that when the valley of the ripple of (minimum of) the changed output voltage is within an allowable voltage range of the load Z, the operation of the load Z becomes unstable and this may cause a malfunction or an unexpected accident.
As an example of using the solid state relay, there is a case where a three-phase load is controlled as shown in FIG. 56. That is, M.sub.1, M.sub.2 and M.sub.3 denote three solid state relays. A three-phase power source 533 is connected to one of the pair of load terminals 6 of each of the solid state relays M.sub.1 to M.sub.3. A heater 534 as a load is connected to the other load terminals 6. In the case of controlling the heater 534 by one input operating control circuit 530 having a DC power source 531 and an operating switch 532, the input operating control circuit 530 is connected in parallel to each input terminal 5 of the relays M.sub.1 to M.sub.3. However, in the foregoing conventional construction, the input operating control circuit 530 must be individually wired to each input terminal 5 of the relays M.sub.1 to M.sub.3 by lead wires. The wiring works are complicated and large enough wiring work space or the like must be provided.
On the other hand, as shown in FIG. 57, in the case where a plurality of loads, e.g., a first load circuit 560 having an AC power source 561, a switch 562 and a heater 563, a second load circuit 550 having a DC power source 551, a switch 552 and a lamp 553, and a third circuit consisting of the three-phase AC power source 533 and a motor 540 are switched on or off controlled by the single input operating control circuit 530 through the solid state relays M.sub.1, M.sub.2, M.sub.3 and M.sub.4, the wiring works upon installation are also complicated as mentioned above.
In such solid state relays, there is a case where a functional circuit such as an AC/DC converter, voltage detecting circuit, etc. is added as an input circuit to the preceding stage of the input terminal 5. If such a functional circuit is included in the relay main body, a variety of types of relays must be prepared for various requirements of the users. Further, the added functional circuit becomes unnecessary or it is difficult to detach at the time of maintenance or the like. On the other hand, when various kinds of functional circuits are individually constructed as the units, the complicated wiring works must be executed upon installation. For instance, as shown in FIG. 58, there is an arrangement such that the 3-phase AC power source 533 is connected to one load terminal 6 of each of three solid state relays M.sub.1, M.sub.2 and M.sub.3 each having the pair of input terminals 5 and a pair of load terminals 6 and the motor 540 is connected to the other load terminals 6, respectively. In this arrangement, when an AC/DC converter unit 571 and a voltage detecting circuit unit 572 which are connected to the input operating control circuit 530 having an AC power source 531A and the switch 532 are added, as well as the wiring between the AC/DC converter unit 571 and the voltage detecting circuit unit 572, the input terminals 5 of the solid state relays M.sub.1 to M.sub.3 must be connected in parallel with the output side of the voltage detecting circuit unit 572, respectively. Thus, the wiring works are complicated and the space for wiring is large.